Raman scattering is a type of inelastic scattering of electromagnetic radiation, such as visible light, discovered in 1928 by Chandrasekara Raman. When a beam of monochromatic light is passed through a substance some of the radiation will be scattered. Although most of the scattered radiation will be the same as the incident frequency (“Rayleigh” scattering), some will have frequencies above (“anti-Stokes” radiation) and below (“Stokes” radiation) that of the incident beam. This effect is known as Raman scattering and is due to inelastic collisions between photons and molecules that lead to changes in the vibrational and rotational energy levels of the molecules. This effect is used in Raman spectroscopy for investigating the vibrational and rotational energy levels of molecules. Raman spectroscopy is the spectrophotometric detection of the inelastically scattered light.
“Stokes” emissions have lower energies (lower frequencies or a decrease in wave number (cm−1)) than the incident laser photons and occur when a molecule absorbs incident laser energy and relaxes into an excited rotational and/or vibrational state. Each molecular species will generate a set of characteristic Stokes lines that are displaced from the excitation frequency (Raman shifted) and whose intensities are linearly proportional to the density of the species in the sample.
“Anti-Stokes” emissions have higher frequencies than the incident laser photons and occur only when the photon encounters a molecule that, for instance, is initially in a vibrationally excited state due to elevated sample temperature. When the final molecular state has lower energy than the initial state, the scattered photon has the energy of the incident photon plus the difference in energy between the molecule's original and final states. Like Stokes emissions, anti-Stokes emissions provide a quantitative fingerprint for the molecule involved in the scattering process. This part of the spectrum is seldom used for analytical purposes since the spectral features are weaker. However, the ratio of the Stokes to the anti-Stokes scattering can be used to determine the sample temperature if it is in thermal equilibrium.
The Stokes and anti-Stokes emissions are collectively referred to as spontaneous “Raman” emissions. Since the excitation frequency (near infrared) and the frequency of the Stokes (and anti-Stokes) scattered light are typically far off the resonance of any component in the sample, fluorescence at frequencies of interest is minimal. The sample is optically thin and will not alter the intensities of the Stokes emissions (no primary or secondary extinctions), in stark contrast to infrared absorption spectroscopy.
Raman spectroscopy is a well-established technology to determine the presence of trace compounds and their isotopomers down to one part per million levels within a host of mixtures. With Raman analysis, absolute concentrations can be determined, the sparse spectra minimize interferences of overtones and combination lines since they are strongly suppressed.
However, conventional Raman spectrometers can require tuning of the incident laser frequency. Additionally, conventional Raman analyzers can lack the desired sensitivity, require an extensive integration time, be too large and/or be too costly for widespread use. Thus, there is a need in the art for a relatively inexpensive, compact Raman spectrometer capable of improved sensitivity and integration times, and capable of operating at high surrounding pressures (up to 800 bars).